Method and apparatus for use in testing materials



Nov. 11, 1930. R. ESNAULT-PELTERIE 7 5 METHOD AND APPARATUS FOR USE IN TESTING MATERIALS Filed April 4, 1927 2 Sheets-Sheet l fl u a/fea Pressure [.2 7

INVENTOR' Y Nov. 11, 1930. R. ESNAULT-PELTERIE 1,781,002

METHOD AND APPARATUS FOR USE IN TESTING MATERIALS Filed April 4, 1927 2 Sheets-Sneak 2 INVENTOR By Attorneys, I

Patented Nov. 11, 1930 ROBERT nsnaun r-rnmnainon rears, runner:

METHCD AND APPARATUS F-DR IIl" TESTING MATERIALS Application filed April l, 1927, Serial No. 180,854, and in Great Britain'February 4, 1927.

This invention relates to a method of. and apparatus for use in testing the physical and electrical properties of materials, and is particularly wel adapted for testing the hardness of materials. The method and apparatus may be used for testing any two objects the mutually opposed surfaces of which are of such form that changes in the pressure causing their contact, result in changes in their mutual contact area. Two spherical objects or a spherical object and an object having a flat surface may be tested together or any two objects the opposed surfaces of which satisfy the aforesaid condition.

This invention has for its object to provide an improved method and apparatus for testing the physical and electrical properties of materials which is more simple. and efficient in use and permits of more accurate determinations, particularly of the hardness of materials, than methods and apparatus heretd fore known.

The determinations made according to the present invention may be either absolute or relative, as will be hereinafter set forth.

That is to say, the hardness of a material, for example, may be determined only relatively with respect to the hardness of another material or absolutely in terms of force per unit v of area required to exceed the elastic limit of the material.

A further object of the invention is to determine the electric contact resistance between specimens of material under various pressures.

Another object of the invention is to provide for the determination of the, mutual contact area between abutting specimens or pieces of material through a determination of the resistance or conductivity of said contact area.

The invention further aims to provide a verysiinple and rapid method for determining the absolute diameter of contact between specimens through a measurement of the apparent diameter of contact in ordinary visible light. The invention further contemplates electrical means for determining with great accuracy the critical point at which a :9 material ceases to follow the law of elasticity,

that is, the point at which the elastic limit of the material is reached. The critical point thus determined is then applied to asgraph which gives preferably with a. high degree of accuracy the diameter of contact for different butting pressures. In this way both a more accurate determination of the critical point and the dianiieterofcontact at such critical point is made. Further objects of the invention will be apparent from the following description.

For convenience and as an example'of the method and apparatus, the invention is hereinafter described in connection with its. application to and use withsp'herical objects in the form of say two balls, as that'is the simplest case, but'the nature of the invention and its application in general will be readily understood therefrom.

According to the present invention the method for testing hardness consists in determining the deformation as indicated by changes in contactarea, these changes being ascertained by observing-the variation in the electrical resistances or conductivities across successive contact surfaces of the material under successive definite and different loads or pressures.

In its preferred form themethod consists in the application of a succession of different and definite loads or pressures to the materials to be tested so as'to produce changes in the mutual area of contact between the said materials, closing an electric circuit across each of the successive contact surfaces, ascertaining the electrical resistances or conductivitiesacross thesaid successive contact surfaces and determining the hardness from the law of variation of the electrical resistances as a function of the load or pressure applied. The apparatus is so constructed and the parts thereof are so arranged that an indication of hardness can be found from the variation of the electrical resistances or conductivities acrossthe different successive contact surfaces of the material when subjected: to definite and different loads or pressures.

In'the apparatus as applied to thetesting of balls, a ball is wedged in a member having a slightly tapered or "conical recess so as to LOU ' give an annulusfof contact between the said ball and the member, the la-tterbein adapted to vbe axially displaced under a de 'nite load oi-pressure so as to cause the said ball to butt against an aligned ball, both balls being in- V *cluded in an electric circuit so that on the passage of an electr c current across the' mu- I tual'contact surfaces between the twoballs theresistance or conductivity offered by the 'said' Contact surfaces can be determined in any known mannerf The success ye values of the electrical resistances or conductivities of the contact surfaces thusobtained 'forrn data in conjunctionwlth the compression forces, from which data the critical POIIIt OI' load-producingthe rupture can be ascertainedr 'This value of the critical load is then transferred to a graph which gives the diam- @eterof mutual contactsurfacesas a function 01" the load or compression forces andfrom ,thisltheahardnes's of the'balls according to Hertzsformula is readily ascerta ned. The

. 2 154,700, "filed Decem g "said method thecontact areas produced by data folr plotting contact areas'as afunction of load may be obtained by anyvsuitable method; such as the method covered by my U S. application, Serial No.

copendin ber 14, 1926. By this various loads are directly recorded on the 'surfaiceof the balls which are first given a veryi'thin coating of material in which the contact area is readily and permanently re corded when the balls are-pressed together. The diameters of the various contact areas resulting from different loads or butting pressures and as recordedinsaid coat ng, are then directly measured with amicrometer microscope. The aforesaid aligned ball is mounted so'as to givean annular line contact in a stationary member which is rotatable at will theother ball being mounted with line contact inthe axially displaceable member and i in axial alignment with the stationary mem- "beer; superposed directly above and in align ment with. the contact surface isa' micro: scepe by; means of which the apparent diameten otthecontact surfaces in visible light can-be ascertained andwhich may be movable and-. adj'ustable vertical-ly in opposite directionscand adjustable laterally or cross-wise otthe apparatus and itdesired also longi- .tu-dinally of, the apparatus. The electric inc-m placeable rm rnber and; the

con nection between an electrical. supply memb" and, the aforesaid axially displaceable be'r i-sort sueh a; nature as to, perm tof epla-cement of the sai di member with imum of friction, and without disconmember For thispurpose a l quid connection of electrically, conductive. nature is proyided between the aforesaid; axially dis e ri upp y terminal to the axially displaceable member and; he two, balls. are included; 1.1 the; election, of; removalof; the electrical supply al circuit Sez hat'curr t l s some supplyterininal through the liquid and the axially displaceable member and from thence across the COIllZfiOt surfaces of the balls. Means are preferablysecuredjtothe casing ofthe apparatus whereby to delivera numher or taps or blows in succession thereto,

the said means being actuatedby vhand or power so'that livered at any desired time and 111 any desired numbe In orderthat theinvention may be clearly understood andreadilycarried into effect reference is made'to the accompanying draw ings wh ch show diagrammatically and by' way of example apparatus'in accordance with the present invention.

Figure l is a diagram showing a graph of the true diameters oi contact all andof the apparent diameters oi'fcontactd' in visible I lght under cliiierent successive loads F. The

value of (Z as a function otF in this particular diagram may be obtained in any suitable manner which forms .no part per se of the present invention. The apparent diameters al in" are obtained by the use of apparatus iniaccordance with the" present invention.

Figure2 is a diagram which gives the dii fer nt corresponding values of the true and apparent diameters d and d f under 'succes sive definite and diiierentloads applied to the-balls. ally very closeto a straightline.

visible light J the taps or blows can be d-e- The curve so determined is gener- Figure 6 is an end view looking in the directionof the arrow A'Figure t. V Figure 7 1s a fragmentarycross-section on;

the lineVIIVII of Fi ure-4 looking in the direction of thearrows In order to facilitate a clear understanding ofthe invention the following explanation is given which considered in conjunction with the graphs-or" Figuresl to 3' and the drawings of the device will enable the method and apparatus of the present invention to be clearly understood, Y r V a ;In' the specification of my prior application for Letters Patent- SerialN 0,154,7(10, filed December 14,, 1926, there is described;

and, claimed a method of and apparatus (hereinafter referred to asthe primary apparatus) for use n obtaining Verynearly an absolute measurement or value of thetrue diameter dot contact between two bodies un-V der successively definite and different com 1 'pression for es thereby e abling. ex:

perimental curve of successive values of cl to be plotted and traced as a function ofF so that it was'thus possible to ascertain up to pression force after whichthe tested mate-- rial ceases to satisfy the'law of elasticity, i.'e., the elastic limit of the material has been exceeded. Such a curve is shown as a dotted line in- Figure 1. It should be clearly understood,however, that it is not absolutely essential that the true diameter (Z of contact should be obtained by the primary apparatus aforesaid, assuch diameters may be obtained by any other known and suitablenianner.

The aforesaid primary apparatus and the main method used therein is slow and to some extent tiring on the operator if used for long periods to make a great number of measurements, so that the measures made after some time are not as accurate as those made at the beginning.- Furthermore, the method contemplated in the aforesaid primary apparatus involves either the use of photography under short wave length light such as the X-ray, or another means for accurately determining the contact area between specimens involving the use of an extremely thin and delicate surface coating upon the balls or specimens tested. l/Vhile both of these methods give very accurate results, they are, as compared with the present method, relatively complicated and expensive, and where agreat number of determinations are to be made the present method greatly facilitates the work it being understood, however, that it is necessary to make use of the methods of my prior j application hereinbefore referred to, or: some other method for accurately. determining the diameter of contact area between specimens in conjunction with the pres ent method in order tobe able to interpret the readings-nowvery simply obtainedby microscopic observation in ordinary light.

In ordinary light it is impossibleto ascere tain the real diameter ofacontact between specimens, but the apparent diameters of a great number of successive contact areas are very, easily obtained. By testing a single specimen according to the said primary methodor'in any other way to determine the true diameters of contact over suitable pressure ranges, a comparison or correction graph may be plotted by which the relationship between true and apparent diameters is established for any given class of specimens; Thereafter a large number of specimens of this class may be tested according to the present method and the true diameters of contact determined by comparing the apparent diameters, as ascertained according to the present invention, with the graph which shows the relationship between true and apparent diameters." A. typical graph of this nature is shown in Fig. 3.. i

Because of the necessity of realizing a practical apparatus and method for rapidly making a: great number of determinations, as wouldbften lie-required in industrial ap plications of myj invention, 1 have been led to conceive an improved apparatus and a, method incidental thereto which avoids practically allcomplication and which actually gives a succession of points which are extremely reliable. Thenew method avoids the necessity of silvering the materials to be tested which is an important simplificationand permits of taking the measurements directly'from the two samplesbeing tested. I

The diameter of contact which is measured according to the new method and apparatus is not the real diameter but an apparent diameter in visible light which is hereinafter referred to as 6Z1. A curve showing such apparent diameters in visible light is shown as a full line in Figure 1, wherein the abscissae represent the successive and different. applied loads or pressures F to which the different apparent diameters d correspond. If the curves cl and al are plotted on the same scale two critical points (X) and (X are necessarily on the same ordinate and the force F producing rupture can be considered in relation to the curve al just as well as to the curve of real diameters. 1 a

Practice has shown, however,that often the critical point (X cannot be accurately appreciated directlyfrom the curve of the apparent diameter d as a function of For this reason according to the present invention aldeterminatio'n is made on a curve presenting the variation of the, electrical confunction of sucessive and increasing values tact resistances B or conductivities as a of F, practice-having shown that on the said curve, as shown in Fig. 2, the critical point (X) is far more obvious and maybe found with greater precision than oneither of the curves (Z or al Theoretically such conductivity should be f proportional to the corresponding real diameter-d'butin fact it is not, because it is diiiicult to measure the resistance of conductivity of the contact area alone. The resistance really measured includes necessarily'some other electrically resistant elements in the circuit. V

It is obvious hat such relative measurements are, however, of great value when combined with corresponding determinations of the absolute values of (Z as a function of F.

On this basis'it is necessary to ascertain first'of all thevariation of the real diameter d as a function of- F byany suitable method fora given kind of mater-iahthe same operations: being, afterwards made on the present vapparatus so as todetermine d as a function of F and under the same successive'loadscr I I "so that. the said handle when rotated is'pre-' 'termining 1 pressures, F fF jFg, etc. as thoseusedin de Having done this it is-possible to trace Ijfor ters d as in Figure 3. v

This beingdone the balls under test are turned in their recesses so as to presenta new f part of their surface to each other and they are-submittedto a succession of definite and increasing compression forces F ,F F etc. which at each t1me 1s definitely andaccurately} known." Under each of these compressive forces two measuresz are made, one with a V microscope and its m crometrical screw to it 3 determined fand the other by anysuitable means to measure the apparent electrical resistance Bacross the contact surfaces. With these three series of values viz. Z'previously f ascertained, andtwo curves are'plot'ted thatwhichshowsthevalues of L as a func- 7 tion of F, Figure 2, shows as already stated a well d'efinedf critical point which gives the valu'e of F corresponding to the elastic That 'valueofF applied to thesecondcurve (see the full'line in Figure 1) 'givesthe value of. al corresponding to theelastic limit.

flTliat'value: of (Zi transferred -t the diagram: -of Figure. '3 previously established: in the manner hereinbefore stated gives the value of the true diameter of contact d'for the e'lasticlimit; V. v I 'f'lhesecrit ic'al or limit-values-of F and-d -are finally.transferredintoFthe Hertz formulee from whichthe co-efficient of elasticity E and the elastic limit of the tested material can be calculatedb s V V '.Referring,more particularly to Figure 4 of the .drawings,'l,j 21*are two 'b alls, both of. which are'of the material to be tested, The

ballis mounted'in a; conical recess-3m a-sta} tionary member4 so as to, give line' contact therein; The stationary member 4; 1s mountedri n asleeve-:-5 whichis rigidly connected 7 l with a surrounding, casingfi; The stations ary member l i-selectrically insulated from the sleeve by insulating material 7 ,andan air o gap 8. The stationary-member 4e is reduced at its outer end as at 9 andfa terminal plate 10 is mountedon thereduced end 9of -the stationary member, '4. The plate 10Tis clamped in 'position by a suitablen-ut 11 between the reduced end' 9. {of the stationary 3 member 4 and an-insulatingwasher 12'. 13, 14 aretwo studs which. are screwedinto the casing 6 andzwhich carry a bridge piece 15 having ELL-recess -16;; in. which is-mounted a handle which: on its inner face f abuts against, the insulating washer 12. "In the bridge piece 15 isa recess-18 with which pins or the equivalent on, the handled? engage, q

vented from axial displacement, by reason of the'engagement of the saidfpins with'the recess18. The stationary member 4 and the handle 17'form a unitary piece which can-be rotated in either of two opposite directions when desired- The plate l0'carries twocontact terminals 19 and 20.

v The ball 2 is mounted in a conical recess 7 f 22 in an axially displaceable member 23 hav- 7 inga reduced outer end 24; with which a rod weights 25 maybe placed in order to'secure the required degreesof loading, The scale beam is mounted upon suitableknife edges so as to transmit the force of the weights to e the rod 25., This rod acts as above described upon'the specimen carrier23' so as to setup the required pressure between the specimens 1, 2. In the absence of weights25 the scale beam and penis preferably perfectly counter balanced by weight25 1 The axially displaceable member 23 is a counter-balancing beam and s mounted centrally in thehollow casing v6 and in alignment with thestationary member 4 sothat the two'balls l and 2are' axially in V alignment; 26 is a pin having'flattened sides 27, 28 (Figures 4 and5), which engage between two ball'obearing guide rollers 27 ,:28 (the lat terbeing visible in Figures) so as to permit of axial displacement of the member 23whilst at the same time" preventing rotation ofthe said'member 23; .29 is arecess con-J taining'liquid mercury '30 into which dips a terminal rod .31 preferablyof coppen'to the upper end of :whichtwo contactterminals 32 and 33 are suitably secured. The axially displaceable member 23 is mountedin the casing 6 byfmea'ns of a suitable longitudinally-arranged ball bearing 85 and the reduced end24 of the axially displaceable member 23*is also movable on a longitudinally-arranged ball bearing 36. V

The casing 6 is provided with a flat bot tomed recess in which is aplate tlwhichcar- IleS- a microscope 42 of any suitable type. The optical axis of the microscope'is positioned directly above-and in the same plane I as the-contact surface of the two ballsso ithattheir profile can be viewed through the microscope. Below thecasing 6 and suitably seclggred thereto 7 is a bracket 43; on which; is

also carries a spindle 50 having a knob 51.

between which and the plate 41 'is an abutment 52 suitably secured to the plate 41. By

rotation of the knob 51, the bracket 46, rack 50 together with the. microscope 42 may be adjusted crosswise of the casing 6.

In testing with the present apparatus it has been found advantageousto subject the casing 6 and through the said casing the interior parts thereof, to a number of taps or blows by means of a tapper or hammer 53 before each measurement of the electric resistance or conductivity. The tapper or hammer 53 is suitably secured to an arm 54 against which a laminated spring 55 bears the arm 54 being carried by a boss 56 mounted on a pin 57. The lower surface of the boss 56 as at 58 is in such a position as to be engaged in succession by the serrations or teeth of a serrated or ratchet wheel 59 mounted on a spindle 60'which is adapted to be rotated by a crank'61 having a handle 62. The serrated or ratchet wheel 59 may have any number of serrations or teeth. Forexample the said wheel may have ten teeth or serrations, each tooth or serration as it comes lnto contact w1th the lever arm 54 causes the latter to rise and consequently the tapper or hammer 53 against the action of the spring 55. Immediately the serration or tooth passes from underv the lower surface of the boss 56 the tapper or hammer-53 drops and strikes the casing 6. The pin 57 andzthe spindle 60'are supported in a plate63 suitably secured to the casing 6.

The resistance or conductivity across the contact surfaces may beascertained by the well known Thompson double bridge method, or by any other method or means which will give sufiiciently reliable indications to detect the occurrence of comparatively small changes in such electrical properties.

The connections of a simple resistance measuring bridge are shown (Fig. 4). In a well-known manner resistances R R R of known value are inserted in the bridge, the upper bridge circuit being completed through theterminals 20, 32 of the apparatus,

as hereinbefore set forth, so that the contact area of the specimens 1, 2 is included in the bridge circuit. A suitable electrical potential is applied by means of a battery B for BX- ample, to the two ends a, b of the bridge and a galvanometer G isconn ected between the midpoints b, 0 of the upper and lower bridge circuits. As is well understood when the values of the known resistances R R R are so chosen as to cause a balance of the aridge resulting in a zero deflection of the galvanometer, the valueof the resistance X, which varies with the resistance of theconv tactareajof the specimens under test, may be determined by the formula The Thompson double bridge method of measuring resistances is preferred because of its greater accuracy. A diagrammatic showing of the Thompson bridge appears in Figure'4 The specimens to be tested are indicated at 1, 2. Acalibrated'resistance slide g, having a fixed contact a and'a movable contact I), is placed in series with the specimens, the circuitv being completed through a battery. B. and controllingswitch S. Between specimen l and thefixed point a, series resistances 11 and T are connected, andtbetween specimens 2 and the slidingcontact 6, series resistances 11 and r; are connected. Bridging thetwo series resistance circuits is a galvanometer G connected at the point 0 between T and T and at the point cl between a and r 7 If R, bethe unknown resistance between the specimens 1 and 2, and R the known resistance between the points a and b of the calibrated slide and the resistances are suitably chosen so as to satisfythe equation Qtfi, then R= R,. I '2 '4 M The value o'f'R is obtained by a-dir'ect scale readingof the position of movable contact b with respect to slide 9.

Theforegoing methods of determiningrresistances' are old in the art, and areset forth merely as illustrative of two suitable ways in which such determinations may be made.

It will be understood that the inverse of resistance values plotted, for example, in Fig. 2, need not be actually ascertained in terms of ohms. Obviously,any series of determinations which hear an ascertainable relationship to theohmic resistance will give the data required to detect the point of discontinuity in-a curve indicating the relationship of resistance and pressure variations.

It will be obvious that the inventionis not limited to the testing of only a single pair of specimens at one time. If one is concerned only with an average determination of the physical properties of several specimens, any number of such specimens could be inserted between the pressure-producing devices and'in the electrical circuit at one time. In this case the pressure would be transmitted from one specimen to another, and likewise, the test current and the sevin series connection.

' A jet of hydrogemnitrogen ornon-oizidiz- 111g gas may be caused to impinge all around physical properties of mater-ials,said appa- I, ratus including a source of electric energy, relatively movable specimen holders, at least one of Whichis energizedbyfsaid electric the contact area during thetaking of measurements so as to prevent oxidation" of the region surroundingthe contact area. For thispurpose, as shown in Fig. '5, a nozzle 65'connected by a tube- 66 'to-gasreservoir 67 may be inserted through the port 68 in the wall of casing 6. Practically the same result is obtained byapplying a thincoating of oil .to-the specnnensiprior'totesting, it,

being understood that in every case inorder to accuratelyiobtain the resistance characteristics of any material the. specimens must have fa bright or polished surface free from corerosion and particles or" extraneous material.

The: application of oil to the surface of the specimens does not injure the accuracy of the determinations made according to the pres- .ent' method; From the foregoing it will be apparentthat in making tests to determine the hardness of material, the absolute value of the contact'resistance between specimens is'of no practicalimportance, it .being necessary only tov ascertain the critical point at which theres'istance curve suddenly changes -its forrny a a l I #Theapparatus may, of course, be used for 'making accurate .determinationof the resistance:between-various specimens of material for purposes other thandetermining f the hardness ofsuch. specimens. 35

' The method and'apparatus, according to the present invention are obviously not limited to the specific disclosures 'hereinbefore made, but may be otherwise variously employed andembodied without departing from the spirit iofthe vinvel ltionas set forthin the following claims.

- .Whatj- I'claim is a H 1 An apparatus for use in testing physical properties of materials, said apparatus in eluding means for Varying by known values the pressure between'pieces of material, an

electric circuit includingthe contact area between said pieces, and electric measuring means sensitive to variations in the electric resistance of said contact area.

2. An'apparatus for use in testing physical properties ofmaterials, said apparatus including means for-.varying the contact area between pieces'ofmaterial by the lbplica 'tionof-a seriesof known pressures thereto,

an electric circuit including the contact area between said pieces, and measuring means sensitive tothe flow of current insaidcirc'ui-t; whereby variationsfin "the conductive properties ofthecontact area between said 3. -.An1 apparatus for use in testing physical properties of materials, saidapparatus including meansfor varying the pressure bene ni p ece f mat ri l, .inechani a l pr ii-grsnooze sure indicating means, 'and; an electric air cuit, the conductivity of' which is varied by the pressure appliedto'said piecesm v 4. An ap 'aaratusfor use 'in testing-th source, said energized holder being provided :with an openingtoreceive' a spherical specimen-ofmaterial, the: said opening: havmg. tapered sides adapted to exert a Wedgng action on the spherical specimen when said specnnenis pressed into the' sa1d open1ng,

whereby an improvedelectrical contact is.

' obtained between said spherical specimen on its holder; i Y

I 5. Theappa-ratus according to claim 4, further characterizedinthat the said energized specimen holder is provi ded with a conical recessof slight taper, within which the spherical specimen is' frictionally held,

v the contact between the specimen and holder forming an annulus ducti'vi'ty.- r

6. An apparatus for use in testing the of good electrical conphysical properties of materials, said appaductiveliquid connection between said. movable holder and one sideuof the current supplywhereby the specimen holder is elec- 7;. Then-apparatus, according to claim; 6, further characterized in that'the said liquid connection comprises, a body of Imercury.-

. .,.l,00 j trified without theuse of a solid connec- 1 84The apparatus according Ito claim 6,

further characterized in that the said speci- 7 men holderfisrecessed' to provide .a well,

mercury in said well,and anyelectrode'pro- I jecting into saidmercury, the saidwell being larger than the electrode to permit of their relative movement.

'9. An apparatus for use in; testing the physical-properties of materials, comprising,

means for pressing with'varying' degrees of Y force, pieces of material together, means adapted to indicate changes in th'ex'elect rical resistanceof the mutual 'contactarea between saidjpieceaand means .for ja'rring said specimens whereby electrical contact "between them. 7

I means for pressing with varying degrees of force, pieces of material together, means to cause a more intimate 1 0. An apparatus, for usejin' testing the physical properties of'materials, comprising adapted to indicate changesin the electrical, 7'

resistance of themutual cont'act area between said pieces, and 111. 31 for delivering a sue cession. Qf tape. r blQ the. p a a s whereby the accuracy of the electrical determination is increased.

11. The apparatus according to claim 2, further characterized in that a micrometer microscope is provided, said microscope permitting measurement of the apparent diameter of contact between the said pieces of material under v rious pressures.

12. The apparatus according to claim 2,

further characterized in that a micrometer microscope provided, said microscope permitting measurement of the apparent diameter of contact between the said pieces of material under various pressures, the microscope being provided with a vertical and two horizontal adjustments whereby its optical axis may be brought into the plane of the said contact area, and the microscope focused on the diameter of contact.

13. The apparatus according to claim 1. further characterized in that means are provided for rotating one of the pieces of material about an axis normal to the said contact area whereby to improve the electrical conductivity of the said contact area.

14. The apparatus according to claim 1, further characterized in that a hammer device is provided, including yielding means causing the hammer head to engage the apparatus and a rotatable actuator adapted to periodlcally move said hammer head away from the apparatus and then release it, whereby said hammer is caused to impart to the apparatus a series of blows or taps.

15. An apparatus for use in testing properties of materials including means for producing variable contact pressures between pieces of material, electrically actuated means for indicating variations in the electrical resistance of the mutual contact area between said pieces, a chamber at least partially enclosing said pieces, and means for supplying to said chamber a non-oxidizing gas whereby to prevent the oxidation of the exposed surface of the said pieces of materials.

16. An apparatus for use in testing the properties of materials, including a normally stationary specimen holder, a movable specimen holder adapted to freely slide toward or away from said stationary holder, a scale beam adapted to transmit various gravitational forces to said movable holder whereby to vary the pressure between two specimens, a micrometer microscope adapted to measure the apparent diameter of the contact areas between the specimens under various loads, and means adapted to indicate changes in the electrical conductivity of the said contact area between specimens.

17. The apparatus according to claim 16,

further characterized in that a tapping device is provided to deliver a series of blows to the apparatus between successive pressure increments.

18. The method of determining the hardness of materials, said method comprising a determination of the variation of the mutual contact area between specimens under definite and oifferent successive loads and a determination of the variation of the electrical resistance of the successive contact areas produced by saidloads, the said electrical data plotted against load rendering apparent the critical load atwhich the elastic limitofthe material is reached, and-this critical ioadgiving onv a graph showing. the relationship between load and contact area, the contact area at which the elastic limit of the material is reached whereby through the application of Hertzs formula the hardness of the material tested is readily determined. a

19. The method of determinin thetrue contact areas under a succession OI different loads between specimens of a given class in the form of balls or the like, said method comprising a determinationof the apparent diameters of contact by direct measurement thereof with a micrometer microscope using visible light, and determining by known means the true diameter of contact over desired ranges of pressure of one specimen'of the said class, and thereafter converting apparent diameters to true diameters by the use of data thus obtained showing the relationship between true and apparent diameters for specimens of said class.

20. The methodof determining physical properties of materials, said method employing the measurement of the electrical resistance between pieces of material, the pieces being so shaped that their mutual contact area varies 'withthe pressure forcing them together. i

21. The method of determining the variable area of contact between two specimens by measurement of tl e electrical properties of said contact.

22. The method of determining the hardness of materials, said method including subjecting specimens of material to various pressures and measuring the resistance of the mutual contact between said specimens while subjected to various pressures.

23. The method of determining the critical pressure at which the elastic limit of a material is reached, said method comprising the observation of the change in electrical resistance of a contact, the conductivity of which varies with the load applied to the material under test. 1

24:. The method according to claim 22, further characterized in that before the resistances of the contact areas are measured, the specimens are subjected to a mechanical jarring while subjected to'pressure whereby to improve the accuracy of the electrical measurements of the resistance.

25. An apparatus for testing the physical properties of materials, said apparatus including mechanical means for varying by lid ' definite 'values the pressure between pieces ofmaterial, said p'ieces being so formed that "their mutual contact area is a function of the pressure, and electrical means adaped to indicate changes inthe resistance of the said mutual contact area. I 7 26yThe method of determining the elastic properties of a plurality of specimens. of material, said method comprising the determina'tion by-known means of the'true diame ter of successive contact areas of one specimen of materialWith-another under a succession of different known pressures, determining under asuccession of pressures the apparent diamete'rsof contact between a series c of specimens of the same class by direct microscopic measurement thereof 'in visible light, ascertaining foreach specimen the pres-i V 7 sure at which its elastic limit is exceeded by 20 observing thejpoint at Which the electrical resistance of the mutual contact area between two specimens suddenly changes, and coniverting the observed apparent dlameters' to truediameters by the data obtained by the elastic limit is reached.-

' first mentioned step in said'method whereby the elastic limit of the several specimens may be calculated in terms-of the pressure and the resultant true diameter of contact area that obtains When the electrically-indicated In witness Whereof,I have hereunto signed my name.

ROBERT ESNAULT-PELTERIE. 

